Add image to README
Fix relative link format
Optimize as_json method - filter requested attributes all at once

- moving adding id into requested attributes to model serialiser;
- support for custom primary_key.
Return back filtering by available attributes separately for each object

- add test for filtering by available attributes for model and collection.
Create accessor for primary_key if it is not id for object serializer

- get primary key from model class for model serializer.
Add ObjectSerializable, include in ActiveResource::Base if defined
Merge pull request #4 from shhavel/master

Add setter primary_key if not id for object serilaizer